In machining an automotive differential housing or carrier, it is necessary to machine two or more coaxial bores of different diameters with at least one bore having a shoulder at one end so that a shaft can be journaled in the carrier. As set forth in greater detail in U.S. Pat. No. 3,884,590, Skrentner, machining of differential carriers requires very close tolerances, accurate coaxial alignment of the two bores and precise location of bearing faces. This generally requires that the bores be machined while the work remains chucked at the same station and preferably by the same boring tool. Moreover, three shafts are journalled in the differential carrier so that close tolerances must also be met as between three shaft journals.
In the adjustable dual tool boring bar disclosed in the 3,884,590 patent, the boring bar is mounted for rotational adjustment with its axis offset from the axis of the machine spindle. Cutting tools are mounted diametrically opposite on the boring bar. Because the boring bar axis is offset from the spindle axis, rotation of the boring bar on its adjustment axis through 180 degrees reverses the tools, one being retracted inwardly and the other being advanced radially outwardly of the spindle axis. Other types of eccentric quills have also been used to machine differential carriers.
Generating heads, also known as facing heads, can also be used to carry a boring bar and move the boring bar radially, transversely of the spindle axis, to perform the multiple machining operations required for differential carriers. However, during machining the boring bar is still eccentric to the spindle axis. Such facing heads may be of the general type shown in U.S. Pat. No. 4,004,332.
Prior art techniques for machining differential carriers using eccentric tooling operated satisfactorily at relatively low speeds of say 250 to 750 rpm with carriers made of steel or cast iron. However, they cannot be used effectively at higher cutting speeds required for cast aluminum housings, for example, 1,000 to 4,000 rpm. With prior art eccentric tools, it is difficult, if not impossible, to maintain the tool balanced and hence maintain close tolerances and accuracies required for precise machining of the differential carrier. The larger internal diameter of the back bore further contributes to the difficulties in maintaining the tool balanced. The tool must have a diameter small enough to allow it to be inserted through the bore with sufficient clearance to allow it to be moved radially to its eccentric position for the back boring operation. With tools extending any substantial distance from the spindle bearings, commonly referred to as overhang, any unbalance in the tooling is further accentuated at the unsupported end of the tool. Even with generating heads where the overhang of the head has been minimized, for example, of the type disclosed in U.S. Pat. No. 4,004,332, the overhang of the generating head and the boring bar contribute further to unbalance and wobble at high cutting speeds.
There is a need for and the objects of this invention are to provide a machine tool and method to perform multiple machining operations such as boring, facing and back boring at high machining speeds; that are particularly suited for machining a differential carrier and the like; that can operate effectively at high cutting speeds required for machining cast aluminum; that are easily adapted to machining bores of different diameters; that effectively utilize a single insert to machine a bore and also face a shoulder at the end of the bore, and that can effectively bore different diameters, all in a single machining cycle.
According to the preferred embodiment of the present invention, these needs and objects are met with three tools that rotate coaxially with their spindle axes. Each tool has a first slide at its free end that carries a first cutting insert for back boring a first diameter and then generating a radial face that serves as a bearing seat on the differential carrier. Each tool also has a second slide carrying a second insert for boring a second diameter. The construction, arrangement and movement of the two slides are correlated with each other to maintain the tool in balance during extension and retraction of both slides. In the preferred embodiment the first insert must be retracted when it is advanced into the carrier, extended to back bore and then retracted to generate a radial face. The second insert is at the same location circumferentially of the tool. One insert is retracted while the other insert is extended, and vice versa, to maintain the tool in balance. In this embodiment, the travel of the first slide to back bore and generate is relatively large. Hence, for similar slides, the movement of the second slide to offset the movement of the first slide is greater than would otherwise be required to bore the second diameter and clear the part when the tool is retracted.
These and other objects and advantages of the invention will be readily understood by one acquainted with the design and use of boring spindles from the following specification and the accompanying drawings.